Adenosine TRI phosphate (ATP) while ADP is adenosine DI phosphate. ATP has a third phosphate group stuck on the end of the molecule. Phosphates are big, bulky moieties, and it is hard to add a third group onto an ADP molecule that already has two. Also, they have a strong net negative charge - so that makes it doubly hard to push negative charges together. So the bond between the third phosphate group and the rest of the ATP molecule is very high energy - it takes a lot of energy to form it. A